Speciation is one of the major challenges present in analytical chemistry.sup.1. Of particular interest is metal speciation. For example, while Cr(III) is essential to our bodies and part of our daily diet, Cr(VI), i.e., chromate, is highly toxic to human causing gastrointestinal disorders, dermatitis, uncertain of skin and is a suspected carcinogenic agent. Thus, the determination of trace levels of Cr(VI), that are often below 1 ppb in natural waters and in biological fluids in the presence of relatively high concentrations of Cr(III) is of particular importance. Although the redox speciation of chromium has been accomplished by separate preconcentration of Cr(III) and Cr(VI) fractions using chelating resins, coprecipitation, ion chromatography and solent extraction, such procedures are obviously complicated.sup.2. Electro-analytical methods that are potentially sensitive for redox-specification have also been used to determine chromate.sup.3. Although Cr(VI) has been preconcentrated and determined on mercury and chemically modified solid electrodes, most of these interfaces exhibit moderate stability and selectivity when employed in natural samples.